For the adults: Studies are limited and inconsistent. One research does suggest that adults laugh an average of 17.5 times per day (Martin RA, Kuiper NA. Daily occurrence of laughter: Relationships with age, gender, and Type A personality. Humor: International Journal Of Humor Research 1999; 12 (4): 355-84.)
Both adults and children laugh primarily during social interactions with others. Consequently, the frequency of laughter at any age depends on how much time an individual spends interacting with others.
How rumors spread: true origins of a false scientific quote
It appears to have all started with Dr Michael Titze, a German Psychotherapist and pioneer in the world of therapeutic humor. Here is what Dr Titze himself had to say about this misquote: